1) Create a new class definition that extends Level. You must implement one of the protected constructors, and call the super constructor within this constructor.
2) Create an instance of this new class using the constructor defined in (1), be sure to set its level value as needed (see the API for Level for the values defined in the default Level classes).
3) Set the level of the Logger using this custom level.
Try the above, and if you get stuck post back with all relevant code.